He was apprenticed to John Lovekyn, a member of the Fishmongers' Gild, and succeeded his master as alderman of Bridge ward in 1368, becoming sheriff in 1370 and lord mayor in 1374.
He became a sheriff in 1370 and lord mayor in 1374. He is said to have suppressed usury in the city during his term of office as mayor. His name frequently figures as advancing loans to the king, and he supported John of Gaunt, duke of Lancaster, in the city, where there was a strong opposition to the king's uncle.
In June of that year, when Tyler and his followers entered south Loudon, Walworth defended London Bridge against them; he was with Richard II when he met the insurgents at Smithfield, and assisted in slaying their leader (see Tyler, Wat), afterwards raising the city bodyguard in the king's defence; for which service he was rewarded by knighthood and a pension. He subsequently served on two commissions to restore the peace in the county of Kent.
He died in 1385, and was buried in the church of St Michael, Crooked Lane, of which he was a considerable benefactor.
Sir William Walworth was the most distinguished member of the Fishmongers' Gild, and he invariably figured in the pageants prepared by them when one of their members attained the mayoralty.